For example, a crankshaft has a journal pin and a crank pin, and a larger end portion of a connecting rod is rotatably supported at the crank pin, and the journal pin is rotatably supported at a journal bearing of an engine block. In order to avoid interference with a mated member, grooves in a cross-sectional semicircular shape (fillet portion) are formed at corner portions of the crank pin and the journal pin. The fillet portion is a portion having low strength, and a large bending moment is applied to the portion when there is an explosion in the internal combustion engine. Therefore, it is desired that a pin top portion (positions at 0 degrees phase and 180 degrees phase in the case of an inline four-cylinder engine) of in the fillet portion be strengthened.
As a strengthening method, a technique for strengthening using work hardening, in which a fillet portion is subjected to a plasticity processing by rotating a roll, is well known. However, the above strengthening method strengthens the entire circumference of the fillet portion, containing portions that do not require strengthening, and therefore, the time and effort for processing are uneconomical. Furthermore, the method simultaneously subjects the entire circumference of all fillet portions on the crank pins and the journal pins to plasticity processing, and as a result, the crankshaft is extended in a shaft direction and is bent.
Patent Publication 1 discloses a technique in which a crankshaft formed by hot forging is flatly placed on a lower die, and then a crank pin is partially subjected to cool coining by lowering an upper die. In this technique, protrusions are formed in each recess of the lower die and the upper die corresponding to the crank pin, and the fillet portion is subjected to a plasticity processing while holding it by the protrusions of the upper die and the lower die, and therefore, a crankshaft is strengthened in a short time.
Patent Publication 1 is Japanese Unexamined Patent Application Publication No. 2004-243373.